William E. McMahon

High-efficiency GaInP∕GaAs∕InGaAs triple-junction solar cells grown inverted with a metamorphic bottom junction

The authors demonstrate a thin, Ge-free III–V semiconductor triple-junction solar cell device structure that achieved 33.8%, 30.6%, and 38.9% efficiencies under the standard 1sun global spectrum, space spectrum, and concentrated direct spectrum at 81suns, respectively. The device consists of 1.8eV Ga0.5In0.5P, 1.4eV GaAs, and 1.0eV In0.3Ga0.7As p-n junctions grown monolithically in an inverted configuration on GaAs substrates by organometallic vapor phase epitaxy. The lattice-mismatched In0.3Ga0.7As junction was grown last on a graded GaxIn1−xP buffer. The substrate was removed after the structure was mounted to a structural “handle.” The current-matched, series-connected junctions produced a total open-circuit voltage over 2.95V at 1sun.

Quadruple-Junction Inverted Metamorphic Concentrator Devices

We present results for quadruple-junction inverted metamorphic (4J-IMM) devices under the concentrated direct spectrum and analyze the present limitations to performance. The devices integrate lattice-matched subcells with rear heterojunctions, as well as lattice-mismatched subcells with low threading dislocation density. To interconnect the subcells, thermally stable lattice-matched tunnel junctions are used, as well as a metamorphic GaAsSb/GaInAs tunnel junction between the lattice-mismatched subcells. A broadband antireflection coating is used, as well as a front metal grid designed for high concentration operation. The best device has a peak efficiency of (43.8 ± 2.2)% at 327-sun concentration, as measured with a spectrally adjustable flash simulator, and maintains an efficiency of (42.9 ± 2.1)% at 869 suns, which is the highest concentration measured. The Voc increases from 3.445 V at 1-sun to 4.10 V at 327-sun concentration, which indicates high material quality in all of the subcells. The subcell voltages are analyzed using optical modeling, and the present device limitations and pathways to improvement are discussed. Although further improvements are possible, the 4J-IMM structure is clearly capable of very high efficiency at concentration, despite the complications arising from utilizing lattice-mismatched subcells.

Inverted GaInP / (In)GaAs / InGaAs triple-junction solar cells with low-stress metamorphic bottom junctions

We demonstrate high efficiency performance in two ultra-thin, Ge-free III–V semiconductor triple-junction solar cell device designs grown in an inverted configuration. Low-stress metamorphic junctions were engineered to achieve excellent photovoltaic performance with less than 3 × 106 cm−2 threading dislocations. The first design with band gaps of 1.83/1.40/1.00 eV, containing a single metamorphic junction, achieved 33.8% and 39.2% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 131 suns, respectively. The second design with band gaps of 1.83/1.34/0.89 eV, containing two metamorphic junctions achieved 33.2% and 40.1% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 143 suns, respectively.

Measuring IV Curves and Subcell Photocurrents in the Presence of Luminescent Coupling

High quality, direct-bandgap solar cells emit significant luminescence at their band-edge when forced to operate in forward bias, thereby creating a possible source of photocurrent in lower bandgap junctions of a multijunction cell. We study the effects of luminescent coupling on the measurement of the subcell photocurrents for a series-connected III–V multijunction solar cell. We describe a technique that uses a set of LEDs and a Xenon-lamp white-light source to accurately determine the subcell photocurrents under a reference spectrum, taking the luminescent coupling current into account. The technique quantifies the luminescent coupling efficiencies and compensates for any spectral overlap between the LEDs and the other junctions. Since quantum efficiency curves are used in the adjustment of the simulator spectrum, we also show how to correct those curves to remove the effects of luminescent coupling.

Monolithic, Ultra-Thin GaInP/GaAs/GaInAs Tandem Solar Cells

We present here a new approach to tandem cell design that offers near-optimum subcell bandgaps, as well as other special advantages related to cell fabrication, operation, and cost reduction. Monolithic, ultra-thin GaInP/GaAs/GaInAs triple-bandgap tandem solar cells use this new approach, which involves inverted epitaxial growth, handle mounting, and parent substrate removal. The optimal ~1-eV bottom subcell in the tandem affords an -300 mV increase in the tandem voltage output when compared to conventional Ge-based, triple-junction tandem cells, leading to a potential relative performance improvement of 10-12% over the current state of the art. Recent performance results and advanced design options are discussed

Evaluation of NF3 versus dimethylhydrazine as n sources for GaAsN

Presented at the 2001 NCPV Program Review Meeting: Hydrazine, NF{sub 3}, dimethylhydrazine, trimethylgallium, and triethylgallium are studied for the growth of GaAs{sub 1-x}N{sub x} by metal-organic chemical-vapor deposition (MOCVD). NF{sub 3} is shown to incorporate nitrogen very much like hydrazine, both of which are more efficient nitrogen sources than di-methylhydrazine. The choice of gallium precursor (triethyl-gallium or trimethylgallium) also affects the nitrogen incorporation. The growth rate of the GaAsN is decreased at low temperatures when trimethylgallium is used and at high temperatures when NF{sub 3} is used. The carbon and hydrogen impurity levels are relatively unaffected by changing nitrogen precursors, but the use of triethylgallium decreases the carbon contamination. These lower carbon levels are not correlated with a significant change in the background hole concentration except for x < 0.2%. The photoluminescence and Hall data for as-grown GaAsN are also unaffected by the choice of nitrogen precursor.

Generalized Optoelectronic Model of Series-Connected Multijunction Solar Cells

The emission of light from each junction in a series-connected multijunction solar cell both complicates and elucidates the understanding of its performance under arbitrary conditions. Bringing together many recent advances in this understanding, we present a general 1-D model to describe luminescent coupling that arises from both voltage-driven electroluminescence and voltage-independent photoluminescence in nonideal junctions that include effects such as Sah-Noyce-Shockley (SNS) recombination with n ≠ 2, Auger recombination, shunt resistance, reverse-bias breakdown, series resistance, and significant dark area losses. The individual junction voltages and currents are experimentally determined from measured optical and electrical inputs and outputs of the device within the context of the model to fit parameters that describe the devices performance under arbitrary input conditions. Techniques to experimentally fit the model are demonstrated for a four-junction inverted metamorphic solar cell, and the predictions of the model are compared with concentrator flash measurements.

Incorporation of nitrogen into GaAsN grown by MOCVD using different precursors

Abstract The incorporation of nitrogen into GaAsN grown by metal-organic chemical-vapor deposition is reported as a function of growth conditions and various combinations of nitrogen and gallium precursors. For all of the precursors, the incorporation of nitrogen is increased by decreasing growth temperature and arsine flow and increasing growth rate. NF3 is shown to incorporate nitrogen very much like hydrazine, both of which are more efficient nitrogen sources than u-dimethylhydrazine. The choice of gallium precursor (triethylgallium or trimethylgallium) also affects the nitrogen incorporation. The growth rate of the GaAsN is decreased at low temperatures when trimethylgallium is used and at high temperatures when NF3 is used.

Surface structured optical coatings with near-perfect broadband and wide-angle antireflective properties.

Optical thin-film coatings are typically limited to designs where the refractive index varies in only a single dimension. However, additional control over the propagation of incoming light is possible by structuring the other two dimensions. In this work, we demonstrate a three-dimensional surface structured optical coating that combines the principles of thin-film optical design with bio-inspired nanostructures to yield near-perfect antireflection. Using this hybrid approach, we attain average reflection losses of 0.2% on sapphire and 0.6% on gallium nitride for 300-1800 nm light. This performance is maintained to very wide incidence angles, achieving less than 1% reflection at all measured wavelengths out to 45° for sapphire. This hybrid design has the potential to significantly enhance the broadband and wide-angle properties for a number of optical systems that require high transparency.

Criteria for the design of GaInP/GaAs/Ge triple-junction cells to optimize their performance outdoors

This paper investigates which reference spectrum should be used to design GalnP/GaAs/Ge triple-junction cells (at 300 K) in order to optimize their performance outdoors (at elevated temperatures). The outdoor performance is simulated using direct spectra from the recently proposed Module Energy Rating Procedure. We find that triple-junction cells designed for AM1.5D, low-AOD and AM1.5G standard spectra at 300 K all work well for maximizing daily energy production at elevated temperatures. AM1.5G cells are the best choice for midday power production, whereas AM1.5D cells are the best choice for power production during the morning and evening. Performance of cells optimized for a newly proposed Low-AOD spectrum is intermediate between these two extremes.